Hydrogen is an essential element for a number of chemical reactions for synthesis at industrial level. It is used, for example, in large quantities for ammonia synthesis (from which all nitrogenous fertilisers are, then, obtained) and in petroleum manufacturing. Some important processes require, furthermore, that the hydrogen is in an ultrapure form (and, in particular, that it does not contain any carbon monoxide, except in traces of less than 10 ppm). Ultrapure hydrogen is, for example, used as missile fuel, for the production of semi-conductor chips and in the metallurgy industry. Furthermore, a considerable use of ultrapure hydrogen has also been foreseen in connection with the widespread use of Polymeric Membrane Fuel Cells (whose catalyst, based on platinum, is made ineffective by impurities, even at minimum levels, present in the feed gas).
Therefore, for a vast range of uses, it is necessary to purify the hydrogen by means of the carbon monoxide selective catalytic oxidation reaction (“PROX” reaction). This reaction requires particular attentions because CO oxidation takes place in the presence of a very high concentration of hydrogen, and the catalyst must be particularly active in removing the carbon monoxide, avoiding, however, in the meantime, hydrogen consumption.
Platinum nanoparticles anchored on an inert support material (such as oxides of aluminium, silicon, titanium or cerium) are generally used as catalysts for said reactions. These catalysts have, however, the disadvantages of the high cost of precious metal, of a relatively high working temperature and of a reduced efficiency (because of the limited selectivity).
Some alternative catalysts have, recently, been developed based on platinum and ruthenium for PROX reaction (S. Alayoglu, B. Eichhorn, Journal of American Chemical Society, 2008, 130, 17479) and based on platinum and cobalt for fuel cells (H. Schulenburg et al., Journal of Physical Chemistry C, 2009, 113, 4069-4077). Said catalysts provide better performances, but they still have the drawback of the high cost due to the use (even if only partial) of platinum in their composition. Furthermore, in the preparation process of such catalysts the Pt component is obtained by using a Pt organo-metallic precursor, which is very expensive and difficult to handle for the manufacturing chemical industry. Accordingly, there is the need to develop an efficient catalyst for the PROX reaction, which combines a good efficiency with a reduction in costs, compared to the state-of-the-art catalysts currently available. In addition, the catalysts that present high activity and selectivity in the carbon monoxide oxidation process have also proven to be catalytic elements active in the anodic and cathodic reactions of polymeric fuel cells as well as in the “Water Gas Shift” reaction, in which the carbon monoxide reacts with water in the vapour state, forming hydrogen and carbon dioxide, in particular, in small and medium-sized reactors.